Abstract
The mechanisms of preferential flow and recharge to groundwater through macropores of sand and gravel aquifer were investigated in the experimental artificial recharge site with recharge pit (2m in diameter and 3m in depth) in Uonuma upland, Niigata Pref. Groundwater dynamics in two experiments of artificial recharge for 10hours/day×5 days were monitored by tracer test, neutron moisture logging and radon concentration measurements in groundwater near groundwater table. Tracer test in the first experiment showed that the groundwater table started to increase after 2.5 hours of artificial recharge and the tracer was detected after 3.5 hours and the concentration of the tracer peaked after 9.5 hours. The second experiment showed as follows. The neutron moisture logging revealed that the parts of increasing water contents with several percent in unsaturated zone gradually moved from shallow zone to deep zone and the downmovement rate of increasing water contents was 0.9m/h in a shallower zone than 6m in depth, and was 2.6m/h in a deeper zone than 6m in depth. The difference of the rates may relate with the difference of the saturated hydraulic conductivity values of two zones. The decreasing radon concentration near the groundwater table gradually occurred from 7 hours after artificial recharge, while the increasing of groundwater level began from 6 hours after artificial recharge. The down-movement rate of preferential flow is three times bigger than the rate presumed from the saturated hydraulic conductivity values. It was assumed that the groundwater level started increasing when the tracer and the recharge water through macropores reached a capillary zone.
